This invention relates to an electromagnetic lifting apparatus, and more particularly to such an apparatus which is capable of tilting or rotating an elongate load about its longitudinal axis.
In many industries which utilize elongate structural steel, such as channels, angles, and I beams, it is frequently necessary to provide access to all surfaces of the steel so as to enable a workman to paint a surface, drill holes, or perform other tasks. This has been accomplished by lifts which electromagnetically engage the elongate load, raise it to a level suitable for the work, and rotate it about its longitudinal axis so as to expose a selected surface. Typically, such lifts comprise an array of lifting elements having electromagnets mounted for rotation about aligned axes and a bipolar or quadrapolar electric motor coupled to each rotatable magnet by a system of pulleys. In order to realize the full lifting potential of the electromagnets, it is essential that their pole pieces be oriented parallel with the surface of the load to be lifted so as to provide a good flux path. If, because of stretching of the pulleys or other reasons, all the pole pieces of each magnet do not contact the surface of the load to be lifted, the total lifting potential of the apparatus is unrealized. In addition, many elongate structures become slightly warped or twisted, and thus, even if all the pole pieces of the electromagnets remain in proper alignment in a single plane, the desired contact between pole pieces and load is not realized because of these surface irregularities.
Another problem with these prior art devices results from the difficulty of synchronizing the rotation of the individual electromagnets. Unless each is synchronized with the others so that all the magnets will rotate to the same degree simultaneously, the apparatus cannot utilize the total amount of torque available for tilting. In this circumstance, some magnets may bear a larger share of the load than others, leading to an ineffective use of the available holding power. Thus, if a single, large motor is used as in certain prior art systems, complicated and expensive means for equally distributing the rotation to the separate magnets must be provided.